Suspended ceiling grids are widely used in commercial and even some residential buildings as they allow ready access to services such as air conditioning, wiring and plumbing that are located in the ceiling space. They are particularly advantageous in multi-story buildings as they allow access whilst minimizing ceiling depth.
If seismic movement was not an issue a ceiling grid could be constructed using only fixed wall angles with main tees and cross runners being fixedly attached by simple means such as fixed angle brackets, to either side of the grid extent in any building area.
To allow for some movement of opposing walls a grid can be made with the tees and cross runners attached at one end only with the free end resting upon a wall angle. In high earthquake areas a 50×50 mm wall angle is typically used to facilitate necessary grid movement. These wall angles are unsightly and traditionally unacceptable architecturally. The free end of the runners resting upon the angle produces an uneven ceiling surface that provides a harbour for dirt and bacteria. Such an arrangement is clearly unsuitable for use in clean rooms or medical facilities where a high degree of cleanliness and hygiene is required.
To maintain structural integrity during seismic events suspended ceilings incorporate 5 way bracing support at very regular intervals i.e. 4 m.times.4 m or 3.6.times.3.6 m. The braces attach the joints between cross members to the ceiling proper and in doing so significantly congest the ceiling cavity.